This invention is concerned with coatings adapted to significantly improve the elevated temperature corrosion resistance of articles composed of iron-, cobalt- or nickel-based superalloys whereby more satisfactory performance and longer life for such articles can be obtained.
Elevated temperature exposure of metal articles is experienced in many situations. Metal components are subjected to such conditions, for example, in various aerospace applications and in land and marine operations such as in the case of blades, vanes, seals and other components utilized in gas turbine engines.
In such applications, it is important to provide some means for preventing undue oxidation/sulfidation of the components involved since such corrosion can materially shorten the useful life of the components. Deterioration of components can also create significant performance and safety problems.
Various alloys, including most superalloys, are characterized by a degree of corrosion resistance, however, such resistance is significantly decreased when unprotected superalloy components are exposed at the operating temperatures involved in certain systems. For that reason, such components have been provided with coatings, such as aluminide coatings, which increase the corrosion resistance at elevated operating temperatures.
Aluminide coatings are applied by methods such as the pack cementation process. In this process, the substrate chemistry and the processing temperature exert a major influence on coating chemistry, thickness and properties. Specifically, the coatings comprise a hard, brittle outer layer and a hard, brittle multi-phase sublayer that can crack when subjected to mechanically or thermally induced strain. This leads to poor fatigue properties, and the cracks can also materially reduce the corrosion resistance of the coated components.
Another class of coatings is the MCrAlY overlay coatings where M stands for a transition metal element such as iron, cobalt or nickel. MCrAlY coatings have been shown to have an advantage over aluminide coatings in providing extended life to turbine components. Specifically, MCrAlY coatings generally demonstrate greater corrosion resistance than aluminide coatings and also greatly superior ductility.
Presently, these MCrAlY coatings are applied by vacuum physical vapor deposition. However, the fundamental nature of the vacuum physical vapor deposition process limits the composition of the coating that can be applied to an article. Specifically, with a single multi-element source, it is very difficult to deposit MCrAlY type coatings which contain other elements that have either very low or very high vapor pressures. Resorting to dual or multiple sources introduces a further degree of complexity to an already complex process which is undesirable from a production standpoint.
Another technique of applying MCrAlY coatings is plasma spraying. In plasma spraying, the heated alloy particles corresponding to the desired coating composition are impinged on the preheated surface of the metal article at very high velocity and temperature. Such particles, upon contact with the metal article surface or with other applied particles, deform plastically and fuse and bond to the surface or to the other applied particles, thus producing a dense and adherent coating. Plasma spraying is particularly desirable since it is a generally less costly technique for producing the overlay coating and is not restricted by vapor pressure limitations as is the case with the vacuum physical vapor deposition processes.
Other attempts at improving elevated temperature corrosion resistance are described in U.S. Pat. No. 4,145,481, issued on Mar. 20, 1979. This process involves the location of a MCrAlY coating over a substrate to provide an overlay, and an aluminide coating was then added as an outer layer. This technique attempts to achieve the advantages of the ductility of the MCrAlY and the resistance to elevated temperature corrosion of the aluminide. Copending application Ser. No. 847,253, filed on Oct. 31, 1977, takes the approach of utilizing first and second MCrAlY-type coatings on a substrate. A first coating is intended to provide a ductile layer with the second coating providing a layer having a greater resistance to elevated temperature corrosion.
Still other approaches, particularly from the standpoint of alloying ingredients and application techniques are described in the following U.S. Patents:
______________________________________ Inventor U.S. Pat. No. Date Of Issue ______________________________________ Gedwill, et al. 3,849,865 Nov. 26, 1974 Gedwill, et al. 3,869,779 Mar. 11, 1975 Hecht, et al. 3,928,026 Dec. 23, 1975 Bessen 3,957,454 May 18, 1976 Preston 4,005,989 Feb. 1, 1977 ______________________________________
In view of the fact that increasingly greater demands are placed on performance, particularly for components subject to extreme temperature conditions, it is desirable to provide even greater improvements in the capabilities of coatings of the type described. The demand for requisite ductility while maintaining resistance to the corrosive effects of temperature and atmosphere is particularly critical.
Oxidation-sulfidation resistance and thermal fatigue resistance at temperatures above 1400.degree. F. is of great importance. Coatings suited for metal components which are subjected to a relatively low temperature (less than 1400.degree. F.) corrosive environment are, however, also of great value.
The low temperature corrosive environment, in particular, refers to the conditions that exist in liquid-fueled turbines burning fuel high in sulfur and vanadium content and operating in a marine environment. Substantial sulfidation (hot corrosion) has been observed in these types of engines, especially when they are operated at low power settings (low temperature). SO.sub.3 has been identified as an agent that can be responsible for this type of attack.
In such applications, it is important to provide some means of preventing the catastrophic corrosion since such corrosion can materially shorten the useful life of the components. Deterioration of components can also create significant performance and safety problems.
Some attempts have been made to develop coating compositions to combat the problem of "low temperature" corrosion occuring below 1400.degree. F. The following U.S. patents describe compositions and application techniques which might be used for this application:
______________________________________ U.S. Pat. No. Date of Issue Patentee ______________________________________ 4,022,587 May 10, 1977 Wlodek 4,088,479 May 9, 1978 Spengler 4,101,715 July 13, 1978 Rairden ______________________________________
These compositions are simply MCrAlY coatings. In view of the energy shortage, resulting in gas turbine engines burning "dirty" fuel containing large amounts of sulfur and vanadium, it is desirable to provide even greater improvements in the capabilities of coatings to provide corrosion resistance at temperatures below approximately 1400.degree. F.